E-mail is a sufficient way of communication for almost every case - just like traditional, physical mail was 100 years ago. You can send a party invite to your friends, you can send family pictures to your mother, you can discuss a project with your co-worker and you can send an invoice to your customer - all of that using a single application. If you need to contact someone immediately - use IM or phone. Why do we need a seperate tool for every scenario? Sure, they provide some useful stuff, but nothing beats simply opening your e-mail client and typing a message to whoever you want.